This invention relates generally to digital radio systems and more particularly to the rapid correction of frequency error in a digital radio receiver which receives communication messages from a transmitter transmitting the communication messages in bursts.
Radio receivers often utilize a circuit to automatically correct for discrepancies in frequency between the carrier frequency of the signal to be received and the frequency of a local oscillator used in a superheterodyne receiver. This local oscillator converts the carrier frequency and associated information carrying sidebands of the received signal to a convenient intermediate frequency. The typical frequency correction process is performed over a relatively long period of time, assuming that the carrier frequency (of the received signal) is continuously present. The carrier frequency may be tracked by means of amplitude detectors, discriminators, or the like to generate a frequency control signal. Some systems may use a pilot signal modulated onto the carrier frequency to provide a reference in the receiver to derive the frequency control signal. (Such a pilot control is further described in U.S. Pat. No. 4,541,118).
The frequency control signal is subsequently applied to the local oscillator from its initial frequency to a frequency which converts the received carrier frequency into an intermediate frequency optimally placed within the selectivity of the intermediate frequency amplification and filtering stages.
Digital receivers present a new set of problems to such conventional automatic frequency control networks. One approach to a fast receiver frequency control for a digital receiver has been disclosed in U.S. patent application No. 285,147, "Digital Automatic Frequency Control on Pure Sine Waves", filed on Dec. 16, 1988 on behalf of Borth et al.
Usually, digital receivers must process the received carrier frequency signal in a linear fashion. Such linear processing allows amplitude variation of the received signal to create further errors in the detection of frequency offset. Furthermore, digital communication is often accomplished using burst transmission techniques such as time division multiple access (TDMA). Burst transmissions do not provide a continuously available carrier or carrier plus pilot which can be employed by conventional frequency control techniques.